1. Field of the Invention
An embodiment of the present invention relates to a semiconductor device having a circuit including a thin film transistor (hereinafter referred to as a TFT) in which an oxide semiconductor film is used for a channel formation region, and a manufacturing method thereof. For example, the present invention relates to an electronic appliance on which an electro-optical device typified by a liquid crystal display panel or a light-emitting display device having an organic light-emitting element is mounted as a component. It is to be noted that the semiconductor devices in this specification indicate all the devices which can operate by using semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic appliance are all included in the semiconductor devices.
2. Description of the Related Art
Recently, active-matrix display devices (such as liquid crystal display devices, light-emitting display devices, and electrophoretic display devices) in each of which a switching element of a thin film transistor (TFT) is provided in each display pixel arranged in matrix, have been actively developed. Since a switching element is provided in each pixel (or each dot), the active-matrix display devices are advantageous in low voltage driving in the case of increasing the pixel density as compared to a passive matrix mode.
A technique for manufacturing a thin film transistor (TFT) or the like using an oxide semiconductor film for a channel formation region and applying it to an electronic device or an optical device has been attracting attention. As examples of such a thin film transistor, a thin film transistor using ZnO for an oxide semiconductor film and a thin film transistor using oxide including indium, gallium, and zinc for an oxide semiconductor film can be given, for example. A technique for forming such a thin film transistor using an oxide semiconductor film over a light-transmitting substrate and applying it to a switching element or the like of an image display device is disclosed in Patent Document 1, Patent Document 2, and the like.
Further, a variety of conductive layers are used for a source electrode layer and a drain electrode layer (hereinafter also collectively referred to as source or drain electrode layers) of a thin film transistor that uses oxide semiconductor. For example, a stacked film of titanium and platinum is known as a metal film (Non-patent Document 1), an oxide including indium (In) and zinc (Zn) is known as a light-transmitting conductive film (Non-patent Document 2), and a stacked film of indium tin oxide and gold is known as a stacked film of a metal film and a light-transmitting conductive film (Non-patent Document 3).
On the contrary, an aluminum film is excellent in conductivity and processability and inexpensive; therefore, the aluminum film is actively applied to a wiring material of a semiconductor element. However, since aluminum atoms are likely to be diffused, it is known that if an aluminum film is simply used as a wiring material, problems due to diffusion of aluminum atoms, such as hillocks, electromigration, or stress migration, are caused. In particular, heat treatment in a manufacturing process of a semiconductor element promotes diffusion of aluminum atoms.
In order to suppress diffusion of aluminum atoms, a method of adding impurities and a method of stacking a high-melting-point metal material such as tungsten or molybdenum are known. In particular, a structure in which a high-melting-point metal material is sandwiched between adjacent layers is effective for enabling diffusion of aluminum atoms to be suppressed. Such a layer including a high-melting-point metal material is called a barrier metal layer.